Perhaps you can make a batch of "chop" and see if he likes that. There are many recipes for it on here and all over the web. I would include more dark yellow meaty or dark green leafy items, kale, mustard greens, sweet potatoes, broccoli, blue berries, seedless grapes, mango, papaya, some spinach, squash, pumpkin, carrots, ect. Just make sure everything you give him is safe.
What seed mix are you giving him? I would cut back on the peanuts as they should only be a rare treat and not very good for birds. Some seeds and nuts are good but not if that's all he eats. I recommend to try Harrison's high potency pellets. If he does not eat them right away (and most don't) you can crush it and put it in with the chop or any fresh fruits/veggies. Fresh is always better! Maybe a trip to the vet for a check up would be a good idea to to know everything you can.
